J . Zool., Lond. ( A ) (1986) 208, 73-82

Play in Harbour seals (Phoca vitulina)

D . RENOUF A N D J . W . LAWSON

Department of Psychology and Marine Sciences Research Laboratory, Memorial University of Newfoundland, S t John’s, Newfoundland, Canada A1 B 3x9

(Accepted 12 March 1985)

(With 2 figures in the text)

Harbour seals in a breeding colony at Miquelon exhibited 13 distinct types ofplay behaviour, 80% of which was solitary locomotor play. Most play was displayed by yearlings and juveniles, though a significant amount of adult play occurred as well. Mothers and pups rarely played. Yearlings played socially as often as alone, but seals of other age classes played by themselves most often. Only social play resembled adult activities. It is suggested that the function ofmammalian play has been obscured by its similarity to adult behaviour.

Contents Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Introduction

Play has been the focus of intensive study in an effort to define it, and discover what function it serves. Young mammals spend a significant proportion of their time playing, most of which emulates adult behaviour (play fighting, play hunting, play copulating and so on). Many authors speculate that it is behaviour which serves no immediate purpose, but functions in various ways to prepare the young for adult activities which are crucial for survival (Aldis, 1975; Bekoff, 1976; Muller-Schwarze, 1978; Lancy, 1980; Fagen, 1981; Smith, 1982, 1984; Brownlee, 1984).

Harbour seals (Phoca uitulina) are like other mammals in that they spend a fair proportion of their time at play (Venables & Venables, 1959; Wilson & Kleiman, 1974; Wilson, 1978). They have been observed playing only on land or in the shallows immediately adjacent to their hauling ledges, though they probably also do so at sea but have been inaccessible for study. These seals forage, compete for mates and copulate at sea. If play functioned to prepare their young for adult activities related to copulation and foraging, one might expect oceanic play to serve this purpose. However, the playing which happens in association with the haul out, where sleeping and nursing are the predominant activities, is more difficult to explain. Other pinnipeds, like the Steller sea lion (Eumetopias jubata), Northern elephant seal (Mirounga angustirostris) and Grey seal (Halichoerus grypus) that feed at sea but copulate ashore, exhibit forms of play on the beach like mock fighting and mock copulating which are similar to adult behaviour. In these instances, the suggestion that it

0022 5460/86/001073 + 10 $03.00/0 @ 1986 The Zoological Society of London 73

74 D. R E N O U F AND J. W. L A W S O N

has a determining role in the development of social behaviour patterns is understandable (Farentinos, 1971; Rasa, 1971; Wilson, 1973; Gentry, 1974). However, a purpose for Harbour seal play near the hauling grounds is not obvious. The following study was undertaken to gain some insight into the play of Phoca uitulina, which might have a bearing on the general problem of play in mammals.

Methods

In a bay in the centre of the French island of Miquelon, more than 700 Harbour seals haul out on sand flats exposed at low tide in the Grand Barachois. The details of this study area have been published elsewhere (Renouf, Lawson & Gaborko, 1983). There, in the spring, they give birth to and wean 150 to 200 pups.

The seals were observed from elevated blinds positioned within 20 m of 3 sites on the sand flats where discrete herds of up to 300 animals hauled out on each ebb tide (Davis & Renouf, unpubl. data). From these vantage points, the age class (pup, weaner, yearling, juvenile and adult) of most seals could be determined. Their sex was usually impossible to ascertain during play.

The group was observed for 120 h from May 23 until July 5,1983, and categories of playful behaviour were identified. In order to obtain detailed behavioural descriptions, a focal subgroup of at least 5 seals was chosen using Altmann’s (1 974) irregular sampling technique, and as individuals disappeared from sight or became inactive, they were replaced in the sample by others. In addition to detailed descriptions of their playing, the following variables were recorded each time play occurred: (1) the age class of the players, (2) whether the play took place in deep water, at the water’s edge, in the shallows covering the hauling grounds before low tide, or on the dry sand, (3) the position of the players with respect to the group settled on the beach, (4) time relative to high tide, (5) the duration of the play bout, (6) whether the play was vigorous, moderate or low in intensity, and (7) the following environmental variables: wind direction and speed, horizontal visibility, the direction of the major water current in the Barachois and sea state. Every 30 min, the distribution of the seals hauled out on the beach was drawn up noting, where possible, the age class and sex of the animals.

Results

Behaviour which exhibited one or both of the following features was considered playful: (1) unusual exuberance and (2) no discernible immediate benefit. Thirteen types of play were identified, which fell under the broader categories of: (1) solitary-individual seals playing apparently alone, or (2) social-two or more animals playing together. This distinction was operational, play being considered social only when we observed two or more seals interacting. This does not preclude the possibility that when only one seal was visible it may have been playing with others who were not.

In 57% of the 345 episodes of playing which we recorded, behaviour in only one of the following 13 categories was involved, while the remainder were sequences which incorporated more than one type of play behaviour. The types of behaviour and their definitions are as follows.

Solitary (1) Porpoisiny. (a) Fast porpoising. The seal swims rapidly, and one or more times leaps clear of

the water by a few centimetres to a metre or more. The seal’s body is oriented horizontally, belly down. (b) Sideroll porpoising. This is similar to fast porpoising, except the seal is oriented on its side. (c) Hammerhead porpoising. The seal swims more slowly than in the other kinds of

P L A Y IN H A R B O U R S E A L S 75

porpoising, comes clear of the water oriented vertically, body rigid with head extended horizontally, and falls back into the water with a large splash (Fig. 1).

(2) Torpedoing. The seal swims very fast through shallow water, hind flippers making large splashes as they drive the animal along, with a prominent wave created by the seal’s back which is often not submerged because the water is so shallow.

(3) Splashing. (a) While afloat, the seal rapidly splashes its hind and foreflippers from side to side at the surface (Fig. 1). (b) Flicking. While lying on its side in a few centimetres of water, the seal repeatedly elevates its hind flippers as far as possible out of the water, in rapid succession or at a steady slow pace (Fig. 1). (c) Thrashing. While lying on its belly or back in a few centimetres of water, the seal rapidly and repeatedly flails its hind flippers and pelvis from side to side causing considerable spray but no forward movement.

(4) Galumphing. The seal moves rapidly across the sand using its foreflippers to raise the body off the sand caterpillar style, the hind flippers sometimes moving from side to side with each forward thrust (Fig. 1).

( 5 ) Digging. While lying on its belly, the seal digs a small depression with its foreflipper and/or nose. After digging is finished, the seal will sometimes repeatedly insert its head into the hole.

(6) Slapping. Each type of slapping (Fig. 1) produces a loud report which is audible for hundreds of metres both in air and under water. (a) Foreflipper slapping. While floating on its side, the seal slaps its abdomen or the water surface with one foreflipper, either once or in a sequence lasting a number of minutes. (b) Hind flipper slapping. The seal floats at the surface and smacks both hind flipper on to the water either once, or as a doublet, or less frequently in a long series. (c) Hind and foreflipper slapping. The seal floats at the surface on its side and first slaps with a foreflipper followed rapidly by a slap with its hindflippers.

(7) Humping. The seal lies on its belly on the sand and rhythmically slides its hind quarters backwards and forwards either rapidly or slowly.

(8) Object manipulation. Seaweed, sticks and other debris are taken into the mouth and chewed and often tossed with vigorous head shaking.

Social (9) Climbing. (a) One seal climbs on another mounting either from the side or the rear. The mount

is brief and is often done repeatedly. (b) One seal climbs on to the back of another and holds on with the foreflippers and usually remains so for a longer period than in 9 (a) (Fig. 1).

(10) RolZing. Two or more seals engage in a series of fluid somersaulting rolls near the surface. Seals usually roll in pairs oriented either head to head as they roll around each other’s longitudinal axis, or the head of one animal will be adjacent to the other’s flippers as they somersault in a cartwheel. Sometimes, rolling is accompanied by heavy splashing as the hind flippers come clear of the water on each roll.

(1 1) Chasing. One seal pursues another either on land or in the water. When the chase is on the beach, one or both seals may move by galumphing.

(12) Mouthing. One seal mouths the head region of another, usually for a short time. (13) Chin sparring. One seal rapidly thrusts its head forward toward another, often dropping the

chin on to the other seal’s shoulder. The mouth is always closed (Fig. 1).

Porpoising occurred most often (25.8%), followed by torpedoing (16.7%). Social play was relatively uncommon, the most frequent type of social play being chasing (4.6%). (See Fig. 2 for a

D. RENOUF AND J. W . LAWSON

. .. --... ... -. - -. . . .._ . .___ .!....' ; ',. . - -. .

FIG. 1 (a) (1) Fast, (2) hammerhead and (3) sideroll porpoising. (b) The three types of flipper slapping: (1) fore, (2) hind and (3) fore/hind. (c) (1) Splashing, (2) flicking, (3) galumphing, (4) climbing and (5) chin spar.

PLAY I N H A R B O U R S E A L S

78 D. R E N O U F AND J. W . LAWSON

Solitary play Social play k 7

w Porpoise

FIG. 2. A frequency distribution of the various types of play behaviour.

frequency distribution of all types of play.) Most play involved relatively intense activity (49% was vigorous, 38.2% was moderately so).

Most playing was done by juveniles (39.8%) and yearlings (36.4%). However, adults accounted for 17.6% of all play recorded and mothers and pups played relatively rarely (1.4%). Weaners were in evidence after 23 June, and their play comprised 1.5% of our observations. Seals of unknown age accounted for the remaining 3.2%. Yearlings exhibited sequences of different types of behaviour almost four times as often as they displayed only one (78.7% vs. 21.3%: Xz = 32.95, d.$ = 1, P < 0.01). Juveniles engaged in sequences (58.5%) about as often as single behaviours (41.5%) (Xz = 2.89, d$ = 1, P > 0.01). Similarly, adults were as likely to play in sequences as not (54.5% vs. 45.5%, X 2 = 0.810, d.$ = 1, P > 0.01). The incidence of play in weaners and mothers and pups was too low to consider in this regard.

Bouts which incorporated only solitary play (80.0%) were more frequent than those which included any social elements (20.0%). The duration of the social play behaviours (X = 23.02 s) was significantly longer than the solitary ones (X = 9.24 s: t = 3.427, d.$ = 236, P d 001). Sequences which included some social play also lasted longer ( j i . = 100.64 s) than those which were completely solitary (X = 21.32 s: t = 3.903, d$ = 75, P d 0.01). Solitary play bouts most often consisted of one to four elements, whereas social play could include longer sequences (see Table I). Of the adult play, 949% was solitary as was all of the weaners’. Juveniles played alone 88.6% of the time, whereas yearlings did so in 56.8% of the cases. When the seals did play socially, they usually played with animals of their own age class (67.1%).

Most playing occurred one to three hours after high tide, at which time there was a shallow layer of water covering the sand flats. As the water level fell during ebbing tide, the seals started to

P L A Y I N H A R B O U R S E A L S

TABLE I Number of’ elements in social and solitary play sequences

Number of elements

79

1 2 3 4 5 6 I 8 9

Social 13 8 I 11 I 5 5 2 3 (%I 19 12 11 16 11 7 I 3 4

Solitary 116 57 43 25 7 9 5 I 2 (%I 42 21 16 9 3 3 3 3 < I

Number of elements

11 12 13 14 18 19 21 24 48

Social 0 2 1 0 1 1 1 1 1 (%I < 1 3 1 < I 1 1 1 I 1

Solitary 1 1 2 1 0 0 0 0 0 (%I < l < l < I < 1 < I < 1 < 1 < 1 < I

congregate. Play occurred as this assembly began and it persisted until the entire group had settled on what eventually became a dry beach. After this time, play was in evidence only sporadically. Of the playing, 36.6% took place in the shallow water covering the hauling grounds as the tide began to fall. However, once there was some dry ground, seals played on that as well (20.173. Of all play, 21.0% occurred at the water line, while 22.3% happened in the deeper water of the Barachois.

The following variables were used in a regression analysis to predict the frequency of occurrence of play in general, and of each type of play: (1) age class of seals hauled out on the beach while playing occurred, (2) date, ( 3 ) duration of play bout, (4) sea state, (5) horizontal visibility, (6) haul out site, (7) number of seals hauled out at the site, (8) wind speed, (9) wind direction and (10) current direction. None of the predictors accounted for a significant proportion of the variance.

Discussion

Most workers recognize at least two broad categories of play. First, there is social play which is a co-ordinated interaction between two or more animals which resembles specific adult social activities. The immediate benefits resulting from the adult behaviour are absent in the play version. The play is structurally different from the adult behaviour in that it is exaggerated in intensity and often scrambles and repeats various components of the adult sequences (Fagen, 1981; Smith, 1982). Some of the Harbour seal play we observed was social, but most was of a second type labelled locomotor (Smith, 1982) or locomotor rotational (Wilson & Kleiman, 1974; Fagen, 1981). This playing (referred to in the vernacular as gambolling, frisking, cavorting, etc.) is characterized by intense movements usually incorporating rotation of the whole body or of particular limbs. It often resembles the movements seen during adult predator avoidance or agonistic chasing, though in play the components may be repeated and disordered. Solitary play is usually of this second type, but locomotor rotational play often involves more than one animal (Fagen, 1981). However, all of the locomotor rotational play of Harbour seals was apparently solitary.

80 D. R E N O U F AND J . W. LAWSON

Both types of play are usually the province of young animals, and if adult play occurs it is often in the form ofmothers playing with their young(Epsmark, 1971; West, 1974; Aldis, 1975; Fagen, 1977; Rasa, 1977; Bramblett, 1978; Pratt & Anderson, 1979; Smith, 1982). However, in the case of Harbour seals, adult play was frequent, though mothers and pups played very little. Similar to the report of Wilson (1978), we found a low incidence of weaner play. However, this is difficult to evaluate since our observations terminated two weeks after the first pups gained their independence.

The resemblance of the play to adult activities is often used as a definitive feature which distinguishes play from other behaviour. This circularity is disconcerting. One might just as easily give the adult behaviour a name, and define it by the fact that it is reminiscent of the behaviour of the young except that it is less intense: its components are often fewer and in a different order, and it has an obvious goal. Defining play by exclusion has led to the development of elaborate theories about its function based, to some extent, upon the fact that the behaviour of young animals is qualitatively like that of adults. It might be more logical to view adult behaviour as a product of the less than perfect behaviour of the young. If young animals who play acquire practice at adult activities, this may be an adaptive consequence of play, but not a cause.

Play may serve a more direct function which has been obscured by its similarity to adult behaviour. In Harbour seals, only the relatively infrequent social play showed this resemblance, being reminiscent of adult copulatory and agonistic displays. Admittedly, social playing may be more common at sea in association with mating interactions. However, their solitary play was, with one possible exception (see below), unlike any non-play adult form we have observed, though it may be similar to as yet undocumented behaviour at sea. Splashing and flipper slapping are felt by some to be part of copulatory displays (e.g. Venables & Venables, 1957; Knudtson, 1977), but this has not been clearly demonstrated. Since most contemporary postulations about the function of play address themselves to social play, they are not relevant to the majority of our observations of Harbour seals. However, some theories are applicable to solitary play. Brownlee (1954) & Fagen (1976) have argued that play gives necessary physical training to the developing animal. On the basis of exercise physiology, Fagen suggested that exercise is most effective in infancy and used this as a partial explanation for why very young animals play. He suggested further that the similarity in form between play and specific adult behaviour ensures that the appropriate muscle groups are in the best possible tone when they are needed for the adult activities that play resembles. His theory cannot yet be confirmed by our observations of Harbour seals playing by themselves for two reasons: (1) solitary play is not clearly like known adult forms, and (2) adult seals played often.

The hypothesis that play contributes to the learning of foraging and predator avoidance skills has been suggested by many authors (e.g. Vincent & Bekoff, 1978; Caro, 1979) including those which have studied pinnipeds (Rasa, 1971; Gentry, 1974; Wilson & Kleiman, 1974). Some elements of object play resembled the movements these seals used to catch fish (Rasa, 1971; elephant seals), or tear it apart (Gentry, 1974; Steller sea lions). However, the object play we observed consisted of behaviour like relaxed mouthing of debris on the beach or casual draping of long strands of algae over their heads, or rubbing along the length of our hydrophone cables. Occasionally, the objects were shaken, but Harbour seals do not shake their prey (Renouf, 1980) as elephant seals reportedly do (Rasa, 1971). None of the chasing of objects blown by the wind as seen in elephant seals was evident in Harbour seals. It might be suggested that torpedoing may have been useful practice for fish catching, in that the seal moved very quickly through the water on a relatively constant bearing as it might sometimes have to do when pursuing fish. However, since Harbour seals learn to forage on their own just after weaning, at the age of one month, it is difficult to see how play which was

P L A Y I N H A R B O U R S E A L S 81

most frequent in animals older than this would add significant fishing expertise. As far as predator avoidance is concerned, we have witnessed porpoising when the seals were forced into the water by humans or when motor boats were close by. It may be that porpoising as play somehow prepares them for this eventuality, a suggestion also made by Wilson & Kleiman (1974). However, Harbour seals have so few predators, it seems improbable that play would have evolved because it reduced the impact of predation. The resemblance of play to adult behaviour might be a ‘red herring’ which is more misleading than helpful. The fact that play and adult activities are similar may simply be a reflection of the fact that all age groups share various forms of the same species typical repertoire.

The predominant feature of Harbour seal locomotor rotational play was its boisterous exaggeration which must have produced extreme vestibular stimulation. Aldis (1975) has argued that this is reinforcing. It has been suggested that animals play because it is pleasurable to do so (Loizos, 1966; Mason, 1967; Bekoff, 1976; Humphreys & Einon, 1981). In most species, the costs of playing (becoming conspicuous, diverting energy from activities important for immediate survival, risking injury, etc.) are too great to allow adults to indulge. However, these costs do not necessarily apply to Harbour seals, which might explain why adult P. uitulina played. These animals are not subject to intense predation and make no attempt to be inconspicuous, injury in water and on sand is unlikely, and their food supply is not seriously limited (Boulva & McLaren, 1979).

It is likely that social and solitary play serve different functions. They were qualitatively different in Harbour seals in that the former lasted longer and incorporated more types of play than the latter. Solitary play was often exhibited as a brief burst of a single play behaviour, whereas social play was more often part of a long sequence. Only the yearlings spent as much time playing socially as alone. It may be, as BekofT(1974) suggested for canids, that social play serves to establish social relationships which persist into adulthood, in which relative strengths and weaknesses and therefore social positions are ascertained and if necessary altered. Perhaps these relationships are established when the seals are yearlings. When the animals played together, they usually associated with their own age class.

The finding that the Harbour seal play we observed was usually solitary, that it was displayed by adults, and that much of it does not appear to resemble other adult activity, renders it somewhat atypical. Further study of this enigmatic species may shed new light on our current understanding of play in mammals.

Summary

Observations of a breeding colony of Harbour seals revealed that, in some respects, these animals do not play like other mammals. Thirteen classes of play near the hauling grounds were identified. Most of it was solitary locomotor exercise, relatively little social play being in evidence. Juvenile and yearling seals played the most, the latter accounting for a majority of the social play we observed. A substantial amount of play was displayed by adults, but mothers and their young played very rarely. Most play did not resemble other known adult activity.

This research was supported by NSERC Grant No. A6364 to D.R. It could not have been accomplished without the continued assistance of the Institut Scientifique et Technique des Peches Maritimes at St. Pierre. Special thanks to Alain Desbrosse of the Departement dagriculture, Miquelon for his logistical assistance. Thanks to Elizabeth Perry for her support in the field and for her comments on the manuscript. Linda Gaborko’s help in the development of this work is, as always, gratefully acknowledged. This is MSRL Contribution No. 584.

82 D . R E N O U F A N D J . W. L A W S O N

R E F E R E N C E S Aldis, D. (1975). Animal play hehauiour. New York: Academic Press. Altmann, J. (1974). Observational study of behaviour: Sampling methods. Behaviour 4 9 227-265. Bekoff, M. (1974). Social play and play-soliciting by infant canids. Am. Zool. 14 323-346. Bekoff, M. (1976). Animal play, problems and perspectives. In perspectives in ethology 2 165-188. Bateson, P. P. G. &

Boulva, J. & McLaren, I. A. (1979). Biology of the harbor seal, Phoca vitulina, in eastern Canada. Bull. Fish. Res. Bd

Bramblett, C. (1978). Sex differences in the acquisition of play among juvenile vervet monkeys. In Social play in monkeys:

Brownlee, A. (1954). Play in domestic cattle: An analysis of its nature. Br. uet. J . 110 48-68. Brownlee, A. (1984). Animal play. Appl. Anim. Behav. Sci. 12: 307-312. Caro, T. (1979). Relations between kitten behaviour and adult predation. Z . Tierpsychol. 51: 158-168. Epsmark, V. (1971). Mother-young relations and ontogeny of behaviour in reindeer (Ranyifer tarandus, L.). Z . Tierpsychol.

Fagen, R. (1976). Exercise, play and physical training in animals. In Perspectives in ethology 2 189-219. Bateson, P. P. G.

Fagen, R. (1977). Selection for optimal age dependent schedules of play behavior. Am. Nat . 111: 395-414. Fagen, R. (1981). Animal play behavior. New York: Oxford University Press. Farentinos, R. (1971). Some observations of the play behaviour of the steller sea lion (Eumetopiasjubata). Z . Tierpsychol. 2 8

Gentry, R. L. (1974). The development of social behavior through play in the Steller sea lion. Am. Zool. 14 391-404. Humphreys, A. & Einon, D. (1981). Play as a reinforcer for maze-learning in juvenile rats. h i m . Behav. 2 9 259-270. Knudtson, P. M. (1977). Observations on the breeding behavior of the harbor seal in Humboldt Bay, California. Calij

Lancy, D. (1980). Play in species adaptation. Ann. Rev. Anthrop. 9 471-495. Loizos, C. (1966). Play in mammals. Symp. zool. Soc. Lond. No. 18: 1-9. Mason, W. (1967). Motivational aspects of social responsiveness in young chimpanzees. In Early behavior: Comparative

and developmental approaches: 103-126. Stevenson, H., Hess, E. & Rheingold, H. (Eds). New York: Wiley. Muller-Schwarze, D. (1978). Evolution of play behavior. Stroudsburg, Pa: Dowden, Hutchinson & Ross. Pratt, D. M. & Anderson, V. H. (1979). Giraffe cow-calf relationships and social development of the calf in the Serengeti.

Rasa, 0. A. E. (1971). Social interaction and object manipulation in weaned pups of the Northern elephant seal

Klopfer, P. H. (Eds). New York & London: Plenum.

Can. 200 1-24.

33-48. Smith, E. 0. (Ed.). New York & London: Academic Press.

2 9 42-81.

& Klopfer, P. H. (Eds). New York & London: Plenum.

428-438.

Fish Game 63: 66-70.

Z . Tierpsychol. 51: 223-251.

(Mirounya anyustirostris). Z . Tierpsychol. 2 9 82-102. Rasa, 0. A. E. (1977). The ethology and sociology of the dwarf mongoose (Heloyale undulata rufula). Z . Tierpsychol. 2 9

82-102. Renouf, D. (1980). Fishing in captive harbour seals (Phoca uitulina concolor): A possible role for vibrissae. Neth. J . Zool.

Renouf, D., Lawson, J. & Gaborko, L. (1983). Attachment between Harbour seal (Phoca vitulina) mothers and pups.

Smith, P. (1982). Does play matter? Functional and evolutionary aspects of animal and human play. Beh. Brain Sci.

Smith, P. (1984). Play in animals and humans. Oxford: Basil Blackwell. Venables, U. M. & Venables, L. S. V. (1957). Mating behaviour of the seal Phoca uitulina in Shetland. Proc. zool. Soc.

Venables, U. M. & Venables, L. S. V. (1959). Vernal coition of the seal Phoca uitulina in Shetland. Proc. zool. Soc. Lond. 132

Vincent, L. E. & Bekoff, M. (1978). Quantitative analyses of the ontogeny of predatory behaviour in coyotes Canis latrans.

West, M. (1974). Social play in the domestic cat. Am. Zool. 1 4 427-436. Wilson, S. (1973). Juvenile play of the common seal Phoca vitulina vitulina with comparative notes on the grey seal

Wilson, S. (1978). Social organization and behavior of harbor seals in Maine. Rep. Mar. Mamm. Comm. No. 76/10. Wilson, S. & Kleiman, D. G. (1974). Eliciting play: A comparative study (Octodon, Octodontomys, Pediolagus, Phoca,

3 0 504-509.

J. Zool., Lond. 199 179-187.

5 139-184.

Lond. 128 387-396.

665-669.

Anim. Behav. 2 6 225-231.

Halichoerus grypus. Behaviour 4 8 37-60.

Choeropsis, Ailuropoda). Am. Zool. 1 4 341-370.